Substation telephone circuits



"Sept. 1,1925. 15513526 A'. B. SMITH SUBS'I'ATION TELEPHONE CIRCUITS Filed Aug. 2, 1921 2 Sh0ets$hee'c l S? x I JED Em?"- fifThur Basaaq E'rfifih Sept. 1, 1925. 1,551,626

A. B. SMlTH SUBSTATVION TELEPHONE cmcurrs viledau z, 1921 2 Shuts-Shut? --Inuan mr Patented Sept. 1, 1925.

UNITED STATES PATENT OFFICE.

ARTHUR BESSEY SMI H, or EVANSTON, ILLINOIS, AssIeNon To AUTOMATIC ELEC- Tmc COMPANY, or CHIQAGO, ILLINOIS, A CORPORATION or ILLINOIS.

'SUBSTATION TELEPHONE CIRCUITS.

application fi1ed August 2,.1921. Serial No.. 489,291." 7

T0 aZZw-Jwm it may concern." I 1 Be it known that I, A THURI Bnssnr. SMITH, :a citizen of the. United States of America, and a resident of Evanston, Cook County, and State of Illinois, have. invented certain new and useful Improvements in Substation Telephone Circuits, of which the following is a specification. .1

' The present inventionrelates in general to sub station telephone circuits, but more especially to substation telephone circuits for automatic. telephone systems; and the object of the invention is to provide ,a novel and improved circuit arrangement 1 for a automatic desk telephone.

In the present circuit a two wire calling device is used in combination with a standard'Bell talking circuit, and the primary object is to rearrangethe parts so as to secure-a minimumnumber of conductors in the cord connecting the telephone proper with the bell box. .In' this respect the in? ventionis, an improvement on the pending application'of Martin Serial No. 449,004, filed March 2, 1921, which in turn is an improvement on the reissue patent to Willis, No. 15,07 0,, granted March 22,1921- In the prior application of Martin referred toonly four conductors are required in the cord.'

It is desirable, however, to further reduce the number of cord conductors, ifthis can be done without a sacrifice of efiiciency in operating characteristics.- The most obviousway to Save another conductor isto bridge the ringer and condenser directly across the line, as is done in party line systems,-. but previous experience has shown that to do this seriously impairs the efficiency of the impulse transmitting circuit, or at least it does so if the ringerand con denser are bridged across the line-alone in the ordinary manner. It has been found, however, that by arranging the circuit 'so that while impulses are beingtransmitted the two induction coil windings in series are connected in parallel with the ringer, the ill effects previously caused by the bridged ringerv and condenser are eliminated, and an impulse transmitting circuit is obtained which is not only perfectly satisfactory, but is actually more efficient under most conditions'met in practice than is a simple series circuit without any bridgewhateven- Shunt circuits are also provided for preventing noises in the receiver while impulses are being transmitted, and for cutting out the transmitter at the same time, and it will beseen then thatwithout anyof the valuable features found in prior circuits being sacrificed, a circuit has been produced for an automatic desktelephone which requires only three conductors in thecord and which at'the sametime includes animpulse transmitting circuit of pronounced efiiciency. In view of the latter feature it. will be understood that the circuit may be used in a wall telephone, or any other situations where the saving'ofcord conductors isan object.

'Havingset forth theobject and scope of the invention in general terms, I will now explain the circui s and apparatus involved in theoperatlon thereof more in detail, reference being had to the accompanying drawings, in which Fig. 1 is a wiring diagram of anautomatic desk telephone and bell box, Fig. 2 is a simplified circuit dia gram, Figs. 3 and 4 are a airing diagram and aschematic diagram, resperively, of a desirable modification, and Big is a schematic diagram of the impulse transmitting circuit.

Referring first to Fig. 2, the talking circuit as before stated is of the well known Bell, common battery type. With the receiver removed from the switch hook, the transmitter T and the winding 4 of the induction coil arebridgedacross the two line conductors 2 and 3, w'hile the receiver R, in series with thewinding 5 of the induction coil and the condenser C, is connected in shunt of the transmitter. The upper and lower branches of the circuit are both normally open and are closed in the'usual manner by the swit-h-hook II when the receiver is removed. Contact springs 6 and 7 are included in the upper branch, while contact springs 8 and 9 are included in the lower branch. j I

. The ringer M which is normallybridged across the line independent of control by the switch hook is provided forreceiving incoming signals. It will be observed that the ringer bridgeincludes a condenser, and that this'condenser is the same one which is included in. the local circuit of the receiver, a, common condenser being used for the. ringer.;and receiver circuits. .A little further consideration will Show that when the bell box.

the contact springs 6 and 7 are closed by the removal. of the receiver the primary winding 5 of the induction coil, the receiver R, and the secondary winding. l of the induction coil, allin series, are connected.

in shunt of. the ringer Theefffect of this shunt circuit will 'be considered more fully later on. 7

Associated with the lower side of the line is a subscribers impulsesender, or. calling device, which is indicated by the reference character S. This sender may be of the type which. is shown and described, onpages and. 35 of Automatic Telephony by Smith and Gampbelhpublished; by the Mo- Grawlfill Book Company. It comprises essentially a. pair. of'impulsesprings 15 and 16 which are controlled by a cam (not shown) through the medium of a finger hold dial (not shown)- and three shunt springs 12, 13 and 14 together with suitable means for retaining these springs in the position shown inthe drawingwhen the calling device is not in use, and for causing all three springs to come into engagement with each other when the calling device dial is turned, from itsnormal position. The impulse springs 15 and 16 are connected in serieswith the lower line conductor. 3 and their function; is to produce momentary interruptions in the line circuit for the purpose of operating the central ofice switches. The function of the shunt springs 12, 13 and 14 is to shortcircuit or shunt the transmitter T and the receiver it while the calling deviceisbeing operated to transmit impulses; To accomplish this spring 13 is connected tothe junction of the receiver and transmitter, spring 1% is connected to the other side of the receiver, while spring 12is connected to the other side of the transmitter, and it will be evident that with the springs connected in this manner when they arebrought into engagement, the receiver and transfer are short circuited as stated.

In Fig. 1 the circuits have been drawn out in detail, corresponding parts being in dicated bythe same reference numerals as in Fig. 2. The apparatus at the right of the drawing, comprising the transmitter T, the receiver R, the switch hook H, and-the call ing device S, constitutes the desk telephone proper; while the apparatus at the left of the drawing and comprising the ringer M the condenser C, and the induction coil 1, is separate from the telephone and in accord ance with the usual practice is located' 'in The telephone is provided with a terminal'strip Pin the base (thereof, and the bell-box is provided with a terminal strip P. The various circuits are completed by meanso-f a cordwhich connectsthe'two terminal strips which, it will be observed, comprises only three conductors. The line extending to the exchange is connected at the terminal strip P as illustrated in the drawing.

The operation of the substation circuit is as follows: hen the receiver is removed to. initiate a call, the switch hook contact springs 7 and 9 come into engagement with contactspringsfi and 8, respectively, whereby the transmitter T is bridged across the linein series with the secondary winding 4 of the induction coil, and the receiver R is connected in shunt of, the transmitter in series withthe condenser C and the primary winding 5 of. the induction coil. Responsive to the closure of the transmitter bridge, current is supplied to. the transmitter from .a bridged battery at the central office (not shown). over the following path: line conductor 2, winding 4 of the induction coil I, switch hook springs (Sand 7, the transmitter T, the impulse springs 15 and16, and the switchv hook springs 8 andv 9 to line conductor 3. Thebridge at the central office, which includes the battery, may also in. accordancewiththeusual practice include the controlling relay of an automatic switch, the

circuit of the. controlling relay including, therefore,the transmitter bridge at the telephone. It followsthat by suitably manipulating the calling device dial the calling subscriber may produce a series of interruptions in the circuit of the controlling relay to-bring about the operation of the switch.

Considering a little more closely theoperations which take place at the telephone duringthe transmission of a series of inr pulses, it will be noticed that as soon as the dial is turned from its normal position the shunt springs are closed together, and the substation loop is completed over a path which may be traced from the line conductor 2 by way-of winding 4- of the induction coil I, switch-hook springs 6 and 7, shunt springs 12, 13, and 1a, the impulse springs 15 and 16, and the switch-hook springs 8 and 9 to the line conductor 3. It will be seen from thisthat the variable resistance of the transmitter is excluded from the switch control, or impulse transmitting circuit, during the entire time that the calling device is in operation, and is included again in the bridge across the line only after the calling device dial returns to. its normal position. At the same time that the transmitter is short circuited the receiver 1% is also. short circuited by means of the shunt springs513 and 14. This latter operation takes care of the usual requirement of silent operation; in other words, with the receiver short circuited the. subscriber is not an- 1 noyed-by hearingthe clicks in the receiver, caused by the interruptions in the line circuit. 'A-n'cther result accomplished by the short circuiting of. the receiver is the reduction of the impedance of the shunt circuit connected around the ringer M. With the receiver cut out,.this shuntcircuit includes only the windings 4L and 5 of the induction coil.

To continue,xtl1e first operation of the calling device brings about the operation of a first selector switch at the exchange, and succeeding operationsvofthe calling device will now result in the operation of additional switches until the connection is finally completed to the line of thecalled subsoriber, itbeing understood that all of the switchesare provided with controlling relay bridgeswhich, with the exception of the bridge in the last switch, are successively cut off from the line after their associated switches are operated. V i

The connection having been completed, conversation may be held'asdesired, the talking circuits functioning the same as usual insuch cases. When the receiver replaced at the end of the conversation, the

upper and lowerbranches ofth'ecircuit are broken at the switch-hook contacts, and further flow of direct currentthrough the substation from the exchange isprevented. As

a result all of the'switches used in the con nectionat the central oflice are released and restore to normal in theusual manner. The substation is now in condition to .receive incoming signalling currents which "may come in over the line conductor 2 and 3 and pass through theringer M and condenser C,

whereby the ringer is actuated to advise the subscriber that. he is being called. 7 A

The impulse transmitting circuit which, as before stated, is exceedingly ei'licient and reliable is shown inschematic form in Fig. 5, .whiclrshows the condition of the circuit through" the substation when the dial is away from its normal position, or in other words,.whil e impulses are being transmitted. The'essential elements are the impulse springs, the permanent bridge including the condenser and the ringer, and the shunt around the ringer which includes windings i and 5 of the induction coil. Since all the non-essential elements, such asthe contact springs, etc., and the elements which are short circuited are omitted in the schematic drawing, the relation between the parts shown may be'perceived at once. It will not be necessary to go into a minute consideration of the operation of the circuit from a strictly technical standpoint, as there are a large number of different line conditions which enter into a theoretical discussion of the electrical properties of the circuit and a thorough treatment of the subject would become exceedingly complicated. .It may be stated, however, that a certain amount of capacitance connected in shunt of the impulse springs is an advantage under a great many line. cond1t1ons,espec1ally 11 there is a considerable amount of loop resistance.

is in series with it, asthis' causes the condenser to charge and discharge very slowly. For nstance, if we consider Fig. Sand assume that the lnductlon COll windings 4 and 5 are omitted when the calling device springs'are closed, the condenser bridge is short circulted and the condenser Wlll be discharged. At each interruption of the 'circuit, the linerelay of the switch at the exchange, if ,it operates properly, should deenergize at once, but with the calling device springs open the condenser bridge is leftv across the line, andthe condenser is immediately charged in series with the line relay. Owing to the fact that this charging circuit also includes the high impedance ringer, the condenser will charge slowly and the'resulting current flow will be prolonged appreciably, thereby causing the deenergi- Zation-ofthe line relay to be delayed. In other words, the effect ofthe condenser and ringer bridge "is" to make the line relay sluggish in responding to interruptions of the line circuit.

Let us consider "now the circuit 'with'the.

inductioncoil windings present, connected as shown 'in Fig. 5. The benefit derived by 7 having the induction coil windings connected in shunt of'the ringer while impulsesare being transmitted seems to be due to the fact that thecondenser is'caused to charge and discharge very much more rapidly. The impedance-of the induction coil windings is relatively low and it will be evident, therefore, that with the circuit of Fig. 5 the condenser will charge and discharge much more rapidly, and the charging current through the line relay will not be prolonged asin the former instance where the induction coil windings were not connected in shunt'of the ringer, with the result that the line relay is 'deenergized much more promptly when its circuit is interrupted. d

Referring. now particularly to Figs. 3 and 4: which as before stated are a schematic and a wiring diagram, respectively, of a desirable modification, it will be noted that the difference between the circuit shown in these drawings and the one shown in Figs. 1 and 2 lies in the arrangement of the switch-hook springs, together with the necessary change in the calling device shunt springs. Specifically, two pairs of switchhook springs are used in Figs. 1 and 2 to open and close the upper and lower branches of the circuit, and as pointed out only three shunt springs are needed to shunt both the receiver and the transmitter. On the other hand, in Figs. 3 and 4. only three switch-hook springs are necessary to open the upper and intermediate branches of the circuit, but two pairs of shunt springs are required to. shunt the receiver and transmitter.

This latter arrangement is very desirable in case instruments already in use that have only three switch-hook springs are being equipped with calling devices, for in that caseit is necessary only to add a calling device that has the required number of shunt springs.

Vhile the explanation might perhaps be further amplified, it is believed that. the foregoing will suffice to enable the invention to be understood and used by, thoseskilled in the art. It will be seen that I have devised an especially advantageous circuit for an automatic desk telephone which includes an impulse transmitting circuit of high efliciency and which in addition has the various parts soarranged that. only three conductors are required for connecting the telephone itself with the bell box.

Having described my invention, I will point out inthe appended claims what I consider to be new and desire to have protected by Letters Patent.

What I claim as my invention is:

1. In a substation telephone set, a line circuit; an induction coil; a transmitter; a calling device comprising a pair ofv interrupter springs; a transmitter bridge across the two conductors. of said line circuit, said bridge including a winding of said induction coil, the said transmitter; and the said interrupter springs; two sets of switchhook springs for closing said bridge, one set being connected between the said induction coil winding and the said. transmitter and the other set being connected between, the said calling device and one line conductor; a permanent bridge across said line, including a ringer and condenser; a receiver and a local circuit therefor including said condenser, a second windingiof said induction coil, the said transmitter, the said calling device interrupter springs, and the second set of switch-hook springs; three. shunt spring closed togetherby the calling device when the same is operated; and circuit connections such that the said shunt springs rupter springs; a transmitter bridge across.

the two conductors of said line circuit, said bridge including a winding of said induction coil, the said transmitter, and the said interrupter springs; switch-hook springs, certain of said springs being connected between the said induction coil winding and the said transmitter for closing said bridge; a permanent bridge across said line includa local circuit therefor including said condenser, a second winding of said induction coil, the said transmitter, the said calling device interrupter springs, and another of said switch-11001: springs; three shunt springs closedtogether by the calling device when the same is operated; and circuit connections such that the said shunt springs short circuit both the transmitter and the receiver via paths excluding said switchhook springs.

3. In a substation telephone set, a ringer and a condenser connected in series and permanently bridged across a line; switch-hook springs; an induction coil; co-operating circuits so arranged that the actuation of said switch-hook springs are effective to connect the windings of said induction coil in multiple with said ringer; a transmitter also connected in multiple with said ringer and condenser when said springs are actuated; a receiver connected in series with said con denser, said transmitter, and a winding of said induction coil, and only one pair. of switchhook springs when said springs are actuated; a calling device provided with interrupter springs for controlling the op eration of automatic switches; and other springs on said device operative during the actuation of said calling device to effectively short circuit both the said transmitter and the receiver regardless of the position of ARTHUR BESSEY SMITH.

mg a ringer and condenser; a receiver and 

